ATTENBOROUGH: The natural world is full of extraordinary animals
with amazing life histories.
Yet, certain stories are more intriguing than most.
The mysteries of a butterfly's life-cycle
or the strange biology of the emperor penguin.
some of these creatures were surrounded by myths and misunderstandings
for a very long time.
And some have only recently revealed their secrets.
These are the animals that stand out from the crowd.
The curiosities 1 find most fascinating of all.
spiders spin intricate webs using their own silk
and birds weave nests from strips of leaves.
1n this programme,
1 investigate the skill of these spinners and weavers
and the way they use such materials
to produce such truly complex structures.
(CHIRPING)
Birds build a variety of nests,
each with a design that is characteristic of their species.
The simplest nests are just sticks wedged into position,
but some are more complicated.
The long-tailed tit builds a delicate nest
from plant material and spider silk
and weaver birds do literally weave with leaves.
But are such skills learned or instinctive?
In 1 905, Eugene Marais, a South African writer and scientist,
was intrigued by the complexity of weaver bird nests.
He wanted to understand more about their nest-building skills
and performed a rigorous but simple experiment
to see if they learnt how to make nests
or if they built them using what he called ''cultural instinct.''
He took eggs from a pair of wild weaver birds
and put them into a canary's nest to hatch.
Then he encouraged the next three generations
of weaver birds to breed, but gave them no nest material
and hatched their eggs once again on the canary's.
When nesting time came for the fourth generation of weaver birds,
he gave them natural nest materials and, without hesitation,
they vigorously set about constructing perfect wild nests.
so nest-building is largely under genetic control,
but it is influenced by experience and the environment.
Nests of the same kind of weaver bird are not always exactly the same,
and the birds, of necessity,
must have some flexibility in how they build.
Nests that hang are particularly difficult to make,
as the birds have to work against gravity,
with no support from below.
Weaver birds solve part of this problem with a skill none others have.
They're the only birds that can tie knots.
These knots vary and are worked on until the weaver succeeds
in attaching several strands of grass to a suitable branch or stone.
These first fastenings are crucial,
as the whole of the completed nest will hang from them.
Once the birds have secured the foundation,
they can start to weave.
Weaving is just one way of binding leaves together.
There are others.
These are tailorbird nests.
They consist of folded leaves stuffed with soft material
and stitched together using spider silk.
The tailorbird pierces the leaves with its sharp beak
and then binds them together by pulling silk through the holes.
The complete operation involves a number of different skills.
Making the holes is like riveting.
Two leaves are placed together
and then pierced to create matching holes above and below.
Then the edges are sewn up.
The upper surface of the leaf
is kept to the outside to help the nest look unobtrusive.
The result is a secure pocket,
which is then stuffed with a soft lining.
The materials the birds choose to sew up their nest can vary.
At the turn of the century,
there was a report in The Common Birds of Bombay
of weaver birds watching carpet-makers and tailors
as they worked on verandas.
When the coast was clear,
the birds flew down and stole tiny pieces of thread
with which to sew up their nests.
(SQUAWKING)
Birds search with a clear idea
of what would be suitable nest material.
Many use sticks and twigs.
(CHIRPING)
They will, however, occasionally use other material
that does the same job
and their choices are sometimes surprising.
This nest was found in an aircraft hangar in the 1 950s,
and is made entirely of twisted wire.
When it was discovered, it contained two blackbird eggs.
1t's an unusual nest for a blackbird,
but similar nests have been found belonging to crows and pigeons.
Weaver birds work with natural material
and, like the tailorbird,
they have to solve the problem of joining leaves together.
After making a knot to secure the basic framework,
they begin their weaving.
They construct the main egg chamber
and then add a small entrance
around the first securely-knotted ring of leaves.
The male, as he works, is under intense scrutiny.
Females are looking for mates, and males that build firm,
well-positioned nests are favoured as fathers.
When he finishes,
a male advertises his handiwork by fluttering.
But he may be forced to build several nests
before a female finally chooses him as a partner.
Weaver birds' nests are very conspicuous.
Other birds, however, go to some trouble to conceal them.
We may not have tailorbirds or weaver birds in Britain,
but we do have long-tailed tits,
delicate little birds that make intricate and finely-constructed nests.
With tiny repetitive movements, they use loops of spider silk
to felt together a mixture of wool and moss.
Both male and female work on the construction.
As their nest takes shape,
they decorate the outside with several thousand tiny flakes of lichen.
The nest is then lined with hundreds of feathers
and provides a delicate but strong structure
to house the growing chicks.
(CHIRPING)
And it's a nest that's particularly hard to find,
because of its covering of lichen.
For years, it was believed
that this acted as a sort of camouflage
to help hide the nest,
but the recent discovery of long-tailed tit nests
covered with small flakes of paper and polystyrene
have helped explain more clearly the reason for this decoration.
Rather than helping to blend the nest with its background,
these small flakes reflect light from it,
making it almost invisible,
and it seems paper and polystyrene do the job
just as well as lichen.
The largest and perhaps the most long-lasting nest of all
is made by the social weaver bird.
(BIRDS CHIRPING)
They live in the dry areas of southern Africa
and work together to build what looks like a great haystack
up in a tree.
New nest chambers are continually added.
As many as a hundred pairs of birds may live together
under the one roof, as you might say.
The chambers provide shade during the day
and keep out the chill at night.
And the whole construction is so robust
that it may provide mass housing
for generation after generation of birds.
Recently, the biggest nest ever recorded
was discovered attached to telegraph poles
in the Kalahari Desert.
1t's more than seven metres across and three metres high.
so weaver birds make their nests in many different ways
and it was once thought that they worked entirely by instinct,
but this is not so.
They are amongst the most expert nest-builders
in the animal kingdom.
And this array of nests shows the complex
and the elaborate designs that they can produce.
Recent studies suggest that weaver birds may be using mental skills
that are not dissimilar to those required to make simple tools.
For weaver birds,
a well-built nest is a ticket to successful breeding.
Who would imagine that such complexity
could be produced using just a foot and a beak?
Weaver birds make their elaborate nests from simple materials
they find around them.
Another of nature's extraordinary builders
are the spiders.
They make their complex webs
from an incredible substance they produce themselves, silk.
Spider silk is unique.
It's very thin, very strong,
and has many exciting potential uses.
spiders spin it with ease,
but scientists have been trying to copy it for many years.
To do that, we need to understand two of the spider's secrets.
The exact structure and nature of their silk
and the way they transform it from a fluid into a thread.
spider silk is a truly remarkable material.
1t can withstand impact and it can be strong, stretchy,
and sticky all at the same time.
spiders produce it from special glands inside their bodies
and extrude it from tiny nipples called spinnerets
at the back end of their abdomens.
And what is more,
they can produce up to seven different kinds,
each with its own purpose.
For centuries, it was the only silk known to man.
The ancient Greeks used cobwebs to stop bleeding
and Australian Aborigines used it to catch small fish.
Then, in the Far East,
a different and mysterious new kind of silk
started to appear and in much larger quantities.
According to Chinese legend,
the first person to weave silk into a fabric
was the Empress Leizu back in the 2 7th century BC.
She was having tea in her garden under a mulberry tree,
when a cocoon fell from the branch above
and dropped into her cup
and started to unravel.
Whether that's true or not,
the Empress Leizu is now honoured as the goddess of silk,
and silk moth farming dates
back to the beginning of Chinese civilisation.
The silk was traded right across the Near East
and into the Roman Empire.
The Chinese traders were sworn to secrecy
about how this marvellous material was made.
But in the year of 532,
the Roman emperor justinian managed to find out
that it came not, as some suspected, from a spider's web,
but from the cocoon of a moth.
silk moth caterpillars produce large quantities of silk
and they make it in a very different way to spiders.
The caterpillars feed voraciously on mulberry leaves
and then, when they're full-grown and ready to transform into a moth,
they spin silken cocoons in which they will pupate.
Unlike spiders, which have specialised spinning organs,
silk moth caterpillars produce silk from their salivary glands.
Each cocoon is made from a single unbroken filament
that can be over 500 metres long.
This silk is plentiful and easy to spin commercially,
but it isn't as tough as spider silk.
And spider silk also has more exciting potential uses.
An orb web like this is constructed over a ''Y''-shaped scaffold
of silk threads, which are extremely strong.
Unlike silkworms, the female spiders,
which spin the webs, are very territorial and aggressive.
So farming it and collecting spider silk is very difficult,
but it has been done.
1n 1 762, a spanish missionary called Termeyer
made a machine that held a single spider
from which he pulled a silken thread.
1n London, Daniel Rolt,
a factory worker, attached spiders to a small steam machine
and succeeded in reeling out 1 8 metres of silk a minute.
That led to machines
which are able to milk several spiders at a time.
Experiments then stopped until 2004,
when two textile artists in Madagascar built a machine
based on these early designs
with which they made something very special indeed.
The golden colour of this stunningly beautiful spider silk shawl
is completely natural.
The silk from which it was made
was produced by 1 ,06 3,000 spiders,
like this one, over four years.
Local people collected 3,000 spiders a day
and trained handlers extracted silk from groups of 24 at a time.
After being milked,
the spiders were released back into the wild.
The individual silk strands were then twisted into a thread,
which was woven into this intricately patterned fabric on looms.
Now, this kind of silk fabric production couldn't work commercially.
Apart from being hard work to make in quantity,
spider silk isn't really a very suitable thread for fabric.
As a cloth, it reacts badly to moisture and heat,
but in its natural state as a single thread,
it has physical qualities that could be exploited medically.
These special characteristics are the consequence
of the molecular structure of spider silk.
1t consists of two large protein molecules.
One is stretchy and spaghetti-like
and the other has a harder, crystalline structure.
Combined, these two proteins give silk
unique qualities of strength and flexibility.
spiders store these proteins as a gel-like liquid in their bodies.
And when they need to make silk,
they extrude it through the spinnerets,
combining the molecules in a special way.
1f we hold down a spider without harming it,
we can see this process in more detail.
Normally, the spider would attach the end
of the silk filament to an object
and then move away,
so that the filament is pulled from her spinnerets.
We can produce the same reaction
by gently pulling the end of the filament itself.
Internally, the silk liquid is passing down a long duct
in which stretchy elements within the protein molecules
are aligned with harder crystalline ones
to create an extremely strong and tough thread.
scanning electron microscopes
reveal how the liquid emerges from the spinnerets.
1ncredibly, spiders can convert liquid proteins
into a hardened thread at room temperature
with very little energy.
1f we could understand and copy this process,
it would be a major scientific breakthrough.
scientists have, in fact, spent many years
trying to replicate the spider's liquid silk
and the way it's spun.
Recently, the genes of spider silk proteins
were cloned and put into goats
to try and produce silk in their milk.
1t worked and, when the goats had kids,
silk proteins were extracted from the mother's milk.
But none of these processes have yet produced silk
that is as tough as natural spider silk.
This machine is called a tensile tester
and it shows how strong and stretchy spider silk can be.
This drag line silk is being pulled apart
and a graph shows the force the fibre is taking
and at what point it breaks.
A steel thread of similar diameter would've broken by now.
There, it's broken.
Spider silk is the toughest natural material known to man.
A single thread of web silk less than a millimetre thick
can absorb the impact of fast-moving prey
and bring it to a halt without breaking.
Complete webs can stretch enormously
and then return to their original shape with a minimum of damage.
1ncredibly, spiders can make this complex material
from just fresh air, flies and water.
The best we can do in making a material like it
requires oil, chemicals, and a great deal of energy.
Although we now better understand the structure of spider silk
and the natural spinning process,
we still can't perform the spider's magic
and copy this extraordinary substance.
But using small amounts of natural spider silk in clever ways
has, nonetheless, a very exciting future.
A sumptuous golden cloth is just one possible product.
This is the dream that has become a reality
and shows just how lovely spider silk can be.
But it also has the potential to make other dreams come true.
1t's a biodegradable material
that we are now using to make artificial joints
and it may even help repair damaged spinal tissue.
This curiosity of nature could eventually save lives.